Thermal-Management Design Banks On Advanced Materials/Adhesives

The electronics industry as a whole relentlessly faces the quandary of unwanted and potentially harmful heat generated by ever-shrinking electronic components. However, dealing with the heat has created heat of another kind—a scorching hot thermal-management market (see “Thermal-Management Market Heads Skyward”).

Miniaturized components in tight packages with high power output create the thermal challenges faced by design engineers. Both the design of a device’s electronics and the device itself must now proceed in parallel, and undergo rigorous computational-fluid-dynamics (CFD) modeling for convective heat transfer and airflow.

Thermal-management needs are expanding in niche markets, too, such as batteries and LEDs. In these cases, increased complexity, density, and intensity have manufacturers searching out new designs and materials for better thermal conductivity, dissipation, and insulation.

Thermal-Management Keys

Several important factors must be considered in any approach to thermal management. They include a good heatsink or heat-pipe design and proper airflow, along with a high, thermally conductive interface material that’s as thin as possible. Also, voids along the interface material need to be eliminated to remove any potential trapped air between the interface material, power-device surface, and heatsink surface.

Supporting high-power ICs on today’s PCBs require working within the limitations of thermal conductivity, coefficient of thermal expansion (CTE), weight, and rigidity. Balancing these different considerations can be tricky. For example, though copper may be very useful in thermal management, it’s unable to manage a board’s CTE, and thus will substantially increase board weight.

Thermal Interface Materials (TIMs)

The miniaturization trend underscores the importance of good thermal interface design, which has ushered in a variety of newly developed thermal interface materials (TIMs). When contemplating a TIM for a particular application, engineers may consider power density, heat dissipation, bond line thickness, processing requirements, and reworkability.

TIMs can be broadly categorized as:

• Polymer matrix composites (PMCs): Different types of carbon fibers combined with a variety of thermosetting and thermoplastic resins, including epoxy, cyanate ester, liquid crystal, nylon, polycarbonate, ABS, PBT, and polyphenylene sulfide.
• Metal matrix composites (MMCs): Silicon-carbide particle-reinforced aluminum, beryllia particle-reinforced beryllium, carbon fiber-reinforced aluminum, copper-tungsten, copper molybdenum, aluminum silicon, and Invar silver.
• Carbon/carbon composites (CCCs): Carbon nanofibers, vapor-grown carbon fibers, nanographene platelets, pyrolitic graphite, and other carbon/carbon mixes.                 

Advanced TIMs, which offer enhanced weight, strength, and thermal-management characteristics, are finding their way into a range of thermal-management solutions. Examples include insulating, hybrid, and nonwoven papers; insulating solders; grease; phase-change materials; and conductive adhesives.

Insulating hybrid and nonwoven papers
Lightweight carbon composite laminates, aramid papers, and nonwovens can be used for heat insulation/dissipation. Easily die-cut to complex shapes, they deliver benefits such as excellent high temperature, abrasion, and chemical resistance; smooth surfaces; high torsional rigidity and stability; and high or low conductivity for absorbing, reflecting, or conducting heat.

Solder
New formulations in solder have expanded thermal control to today’s high-performance, high-energy, and high-heat applications. Examples include lead-free die-attach solder alloys and active solder (lead-free solder with added titanium or rare-earth elements). There’s also eutectic bonding/soldering, where silicone is alloyed with metals, such as gold or aluminum, to enhance heat dissipation/management.

Thermal Grease
The traditional interface material in electronics is thermal grease. Available in silicone or non-silicone varieties, thermal grease offers low thermal resistance through excellent gap filling, and an extremely thin bond line. The reworkable, low-cost material is easily applied (including automated dispensing) and maintains good reliability.

Thermal grease also can be applied as a thermal interface pad, where the grease is impregnated in the pad. The thermal pad offers the same “wetting” capability as thermal grease, but can be die-cut to custom shapes for “drop-in-place” assembly.

Issues may arise with grease, though, such as contamination (for silicone grease), pump-out during thermal cycles, and imprecise or inconsistent application.

Phase-Change Materials
Phase-change materials (PCMs) often supplement some of the issues related to grease. PCMs are solids at room temperature, but change to liquid once a device’s excess heat pushes the material past its melting point.

Typically composed of a coating of phase-change compound on an aluminum or polyimide substrate, new PCMs can be coated directly onto a release liner without using a substrate. This creates a better flow when the PCM is in the liquid stage, and better gap and void filling, all of which ultimately improves performance. The interface is thinner without the substrate, resulting in greater heat-transfer efficiency.

The PCM, which is more “manufacturing-friendly,” doesn’t pump-out of the interface like grease. There’s no messy application or cleanup necessary.  Over the long run, PCMs often prove to be more reliable in terms of thermal management than grease.

Thermally Conductive Adhesive
Adhesives offer unique options for thermal management. Often, they’re the best choice when components aren’t connected by mechanical attachment, or when substrate micro-movement requires adhesion so that a component can maintain contact with the substrate. Many times, these will be used with semiconductor packages as an interface between a chip and a heat spreader.

Thermally conductive adhesives can take a number of forms. They can be conformable interface pads that are easy to handle and provide high conductivity. Another is liquid form, usually epoxies that feature an ultra-thin bond line and can be easily integrated into manufacturing dispensing equipment. A third form is tape, which typically maintains high mechanical strength plus good surface wetting and excellent shock absorption.

Beyond semiconductor applications, these adhesives are popular in automotive electronics for attachment and thermal-management purposes. In addition, adhesives can combine thermal and electrical conductivity, for example, as an electrical ground to a board. Or conversely, an adhesive could be thermally conductive but electrically insulating.

Using an adhesive for thermal management requires considering potential tradeoffs in bond strength versus heat dissipation. For instance, a thick application increases the bond, but decreases heat dissipation. It’s also important to consider how much “filler” is in the adhesive. A lot of filler provides high shear strength, but lower flexibility. In this case, it’s essential to calculate the coefficient of thermal expansion (CTE) between the component, substrate, and adhesive.

Finally, all of the aforementioned possibilities will need to be assessed in terms of suitability for the manufacturing process and cost.

Lean On Experienced Converter/Materials Suppliers

An experienced materials converter and industrial assembly supplier becomes an indispensable component during the process of choosing thermal-management materials for a particular application. From identification and selection of the appropriate materials and adhesives, to slitting, layering, laminating, precision die-cutting, and packaging of the finished product, an experienced converter can assist with design, prototyping, testing, and manufacturing.

An experienced converter, say, can select from servo-driven rotary die-cutting, CNC die-cutting, laser die-cutting, and water jet cutting to meet the complex specifications of thermal management for electronic components. For example, a servo-driven rotary die-cutter will maintain tight tolerances ranging from 0.015 to ±0.005 in. at speeds up to 500 fpm. Thus, it’s ideal for the potentially complex, multi-layer die-cutting, and lamination required by a thermal interface pad or tape.

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For complex foam tape die-cutting, water jet technology creates clean edges with no distortion. Laser die-cutting, kiss-cutting, slitting, and laminating also can be used to convert applications. With a grease or liquid TIM, the converter can provide and plan for easy integration into the manufacturer’s process with dispensing recommendations and solutions.

A converter with a fully equipped test laboratory can verify whether customer materials meet designed-in specifications before they move to the factory floor, often eliminating the need to test materials at the customer’s facility. A complete test lab offers:
 

• Accurate and precise part dimension measurement and verification
• Adhesive/release liner to determine converting properties and high-speed application characteristics
• Material strength measured to ensure that material meets application requirements
• Static shear testing to measure the cohesive strength of the adhesive to withstand a fixed load over time
• Material weight measurement to determine adhesive coating weight
• Microscopic imaging to determine differences between the adhesive and material over time
• Dielectric testing to determine a material’s electrical insulation properties
• Thermal testing for materials and adhesives
• Resistance and voltage testing to profile a material or adhesive’s electrical properties 

Market Update
Thermal-Management Market Heads Skyward

A September 2011 report from the Electronics.ca Research Network revealed that the global thermal-management market will grow from $8 billion in 2011 to $10.9 billion in 2016, with a 6.4% compound annual growth rate (CAGR). Thermal-management hardware, including fans, blowers, and heatsinks, is forecast to balloon to $9.1 billion by 2016, up from $6.7 billion in 2011.

The thermal-interface materials portion of the market totaled $426 million in 2011, and is estimated to reach $627 million by 2016. Within this segment, polymer-based materials will experience the strongest growth rate, from $373 million in 2011 to $549 million by 2016. Metal-based materials will grow from $39.5 million (2011) to $53.8 million (2016), and phase-change materials from $13.4 (2011) to $23.7 million (2016).

The top thermal-management companies developing new technologies, and thus driving the growth, include 3M, Aavid Thermalloy, Amkor Technology, ANSYS, Bergquist, Chomerics, DuPont, Henkel Loctite, Kyocera, Kytron, Metafoam Technologies, Orient Semiconductor Electronics, Phononic Devices, Stats Chippac, Thermacore, Vette, and Wakefield Solutions.